1. Field of the Invention
The present invention relates to a stage apparatus. In particular, the present invention relates to a stage apparatus which is preferably used, for example, as an X-Y stage apparatus included in an exposure apparatus or the like.
2. Description of the Related Art
An exposure apparatus, which transfers a pattern on a reticle with light for exposure onto a substrate to be exposed (photsensitive substrate, hereinafter referred to as "substrate"), includes a means for moving the substrate to a predetermined exposure position. An X-Y stage apparatus, which is provided with a substrate stage capable of movement in X and Y directions, is used as such a means for moving the substrate. Those which are used as such an X-Y stage apparatus include, for example, stage apparatuses of a type which make guidance by using static pressure air in order to realize movement and positioning of a stage at a high speed with high accuracy as disclosed in Japanese Patent Laid-Open Nos. 1-188241 and 4-223830.
FIG. 5 shows an example of a conventional X-Y stage apparatus of this type. In FIG. 5, a pedestal 100 is made of iron, to which an X guide 102 made of iron as a guide means in the X direction, and stators 104A, 106A of linear motors 104, 106 as driving means in the X direction are fixed. Movable elements 104B, 106B of the linear motors 104, 106 are connected to first and second Y guide carriers 110, 113 respectively. The spacing between the Y guide carriers 110, 113 is bridged with a Y guide 108 which serves as a guide means in the Y direction. Namely, the Y guide carriers 110, 113 and the Y guide 108 constitute a movable unit in the X direction. The movable unit is driven in the X direction in accordance with movement of the movable elements 104B, 106B in the X direction.
The first Y guide carrier 110 is provided with air-ejecting ports and permanent magnets (not shown) on its surface opposing to the pedestal 100 and on its another surface opposing to the X guide 102. The second Y guide carrier 113 is also provided with air-ejecting ports and permanent magnets (not shown) on its surface opposing to the pedestal 100. The first Y guide carrier 110 and the second Y guide carrier 113 float over the pedestal 100 and the X guide 102 while maintaining gaps of predetermined distances therebetween in such a manner that the pneumatic pressure as a repulsive force is balanced with the magnetic force as an attractive force.
A Y movable unit 111, which is movable in the Y direction along the Y guide 108, is attached to the Y guide 108 in such a manner that the Y guide 108 is surrounded by the Y movable unit 111 on its upper and lower, and right and left sides. A pair of X direction bearings 112 (one counterpart is not shown) are fixed to both side walls of the Y movable unit 111 so that the X direction bearings 112 interpose the Y guide 108 on both sides in the X direction. Air-ejecting ports are provided on the pair of X direction bearings 112 respectively. Accordingly, pressures of air spouted from the pair of X direction bearings 112 toward the Y guide 108 make a balance which allows a constant gap to be set between the X direction bearings 112 and the Y guide 108.
Air-ejecting ports and permanent magnets (not shown) are provided on a surface of the Y movable unit 111 opposing to the pedestal 100. The Y movable unit 111 floats over the pedestal 100 while maintaining a gap of a predetermined distance in such a manner that the pneumatic force is balanced with the magnetic force.
A stator 114A of a linear motor is fixed on an upper surface of the Y guide 108. A movable element (not shown) of the linear motor is attached to the Y movable unit 111. Thus the Y movable unit 111 is driven in the Y direction integrally with the movable element.
A top plate of the Y movable unit 111 serves as a substrate stage 114. Movement mirrors (not shown) for reflecting light from laser interferometers as coordinate-measuring means are carried on the substrate stage 114. The movement mirrors are moved by the movement of the substrate stage 114 in the direction of X and Y. A substrate is placed through a table (not shown) capable of being controlled in the Z direction and for rotation about X, Y, and Z axes.
It is important for the X-Y stage apparatus that the substrate stage 114 is controlled for its position in a short position-adjusting time, and it is controlled for its velocity in a short velocity-adjusting time. Accordingly, it is desirable that the entire apparatus has a high mechanical resonance frequency. Therefore, it is desirable that the static pressure air bearing has high rigidity in the vertical direction, and the pedestal has a light weight and also has a property that it is not readily flexed due to movement of the substrate stage 114 (more specifically, movement of the X movable unit and the Y movable unit).
However, in the conventional X-Y stage apparatus as described above, those made of iron having relatively heavy weights and small rigidity have been used for the pedestal 100 and the X guide 102. For this reason, the entire apparatus has a low mechanical resonance frequency, and deflection occasionally occurs in the pedestal 100 due to movement of the substrate stage 114. In addition, the pedestal 100 is apt to be scratched. When it is scratched, the scratched portion is rusted, or a bulge called "burr" is generated thereon. Accordingly, there has been an inconvenience that the moving performance of the stage is deteriorated when the amount of bulge exceeds the bearing gap of 5 to 10 .mu.m.
The reason why the pedestal made of iron has been hitherto used is that each of movable members is supported over the pedestal through magnetically pre-loaded air bearings. In the case of the static pressure air bearing, it is important to set the bearing gap to have an optimum value because the rigidity of the bearing gap affects pitching and rolling performance of the movable unit. Thus, it has been hitherto necessary that the pedestal and so on should be made of a magnetic material because the bearing gap of the magnetically pre-loaded air bearing is determined by the balance between the pneumatic pressure provided through air-ejecting ports and the attractive force of magnets attached to the movable unit for attracting the pedestal and so on.
Characteristics of the magnetically pre-loaded air bearing can be controlled by adjustment of magnetic force by replacing magnets, or by change in attachment positions of magnets. Such an adjustment operation can be performed for single parts, however, it cannot be performed after assembling the stage apparatus and the exposure apparatus. Due to this reason, there has been an inconvenience that it is difficult to finely adjust dispersions of bearing gaps resulting from errors in processing of parts or assembling errors.
In addition, when the magnetically pre-loaded air bearing is used, an eddy current is generated by the electromagnetic induction effect in the vicinity of the surface of iron in accordance with relative movement of the magnet and iron. The eddy current and the magnetic force act to generate a force in a direction to stop movement of the movable unit. Accordingly, it is necessary for a driving linear motor to generate a large thrust force, resulting in an inconvenience that the motor necessarily has a large size, the electric power consumption increases, and the amount of heat generation increases as well.
In the conventional X-Y stage apparatus as described above, a single linear motor is used to drive the substrate stage in the Y direction. Accordingly, the apparatus suffers large external disturbance due to thrust ripple when it is controlled at a constant velocity. In addition, the linear motor for driving in the Y direction is located under an optical path for the Y interferometer (not shown) and along the optical path. For this reason, there has been also an inconvenience that an error is caused in the value measured by the interferometer due to temperature-dependent fluctuation in air existing on the optical path resulting from the influence of the increased heat generation even when forced cooling is applied.